The two most common methods for drilling rock involve either quasi-static loading of rock as used in rotary drilling, or high intensity impact loading as used in down-the-hole (DTH) drilling. DTH applications include a hammer assembly having a piston or actuator that reciprocates within the drill casing and applies a cyclical impact on an anvil. The anvil is typically part of or directly connected to the drill bit so that impact forces of the piston striking the anvil are transferred through the drill bit into the rock being drilled. The piston typically reciprocates in response to motive fluid (e.g., compressed air) alternatingly raising and lowering the piston. All motive fluid is typically exhausted from the drill through the drill bit after actuating the hammer assembly. Exhausting motive fluid through the drill bit clears cuttings and other debris from around the drill bit and carries such debris up out of the hole or bore being drilled. Hybrid rock drills (called percussive assist rotary drills or PARD) that utilize a DTH hammer assembly to impact a rotary drill bit are also known, and also exhaust all motive fluid through the drill bit.
When motive fluid is exhausted through the drill bit, it flows over an exterior surface of the drill bit (“flows over” and variations thereof meaning in this specification that the motive fluid flows across and in contact with the drill bit exterior surface) and up the bore being drilled. In known DTH hammer assemblies having reverse circulation configurations, the motive fluid is actually exhausted above the drill bit, flows down over the drill bit exterior, and then flows up through the center of the drill bit, drill assembly, and drill pipe or drill string to the surface. In this specification, the term “through the bit” and “bit exhaust” are intended to include exhausted motive fluid that flows over the drill bit exterior surface, whether flowing out of the bit and up the bore or flowing in a reverse circulation direction.
In the present application, the terms “down hole hammer,” “hammer,” and “hammer assembly” refer to a drilling arrangement using the impact forces of a reciprocating piston or other moving actuator, whether such drilling arrangement is present in a DTH application, a PARD arrangement, or another arrangement, and regardless of whether the drilling arrangement includes a standard bit, drag bit, rotary bit, or another cutting surface.
The present invention relates to a down hole hammer that exhausts at least a portion of the motive fluid through a portion of the drill other than the drill bit. For drilling operations in which the drill bit is at or near the bottom of the drill assembly, the invention may be termed a down hole hammer having a portion of motive fluid exhausted above the drill bit or a down hole hammer having elevated exhaust. The invention also relates to a down hole hammer in which motive fluid is divided into a portion that is exhausted through the drill bit or elsewhere such that it flows over a portion of the drill bit's exterior, and a schematically parallel portion that operates the piston and is exhausted above the drill bit such that it does not flow over the drill bit's exterior surface.